Dome slaving setting for SBS setups

Pretty sure this is not supported from how I understand the slave settings setup works with domes. But is it possible to add a offset value to handle when the centerline between two scopes does not align with dome slit?

I think that to properly account for the geometry of a two-scope setup significantly complicates the calculations of the allowable slit position to avoid intersecting the light rays entering both scopes. For instance, when you’re close to a meridian flip, the scopes will be “vertical” and will be oriented parallel to the slit, so any offset won’t be important, whereas close to the celestial equator, they’ll be “horizontal” and will present the greatest width through the slit in terms of the incoming light rays. I’m sure it’s way beyond the scope of any of the bits of software that I’ve ever used. I think the only way of handling it under these limitations would be to assume the worst case, which would be the equivalent of having a large scope that encompasses the outer edge of both of your smaller scopes, and then make sure that the dome position is updated sufficiently frequently so that the dome never intersects the outer edge of your most offset scope. In SGPro, this means setting the “allowable error” in the “slave settings” to a lowish value. Someone else may have a better solution, but that would be my way of tackling it.

I had a similar problem with my shutter opening and a two scope set-up. I got round it by measuring the widest point between both scopes and using this figure as the diameter of a single scope into my dome setup. Don’t forget to alter in both the horizontal and vertical dimensions


Thanks guys, I think I am following along with the idea of thinking of this as one large scope for setting up the Slave Settings. In that section, is the only value I need to adjust here to this “virtual large scope” the GEM offset? So for instance I am going to be mounting my 8"RC and 61mm refractor on a ADM SBS and from ota edge to edge is 17.375". So I want to think of this as that diameter (or sightly larger) OTA. My nexdome slit opening is 24" so there is a little room on each side. Also I only plan to image with one OTA or the other at any given time. Does that factor into this? Meaning when I do a plate solve, with the slit be center to that OTA even if it is offset from the EQ mounts. I am guessing it will vary depending on where it is pointing and might get messed up after a meridian flip.

I don’t think you need to set the GEM offset so high - I think that can be set to the distance from the RA axis to the optical centre of the largest of your scopes. I believe all you need to adjust is the allowable error. After slewing the scopes and the dome - the centre of the slit should line up with a theoretical telescope centred on the DEC axis. What you will need to do then is look at how far the outer edge of your furthest scope is from the DEC axis - I assume this would be the 61 mm refractor as it would be lighter and so further out to keep things balanced. So, say your 61mm outer edge is 10 inches from the DEC axis, then light entering it would be 10 inches off centre in the slit, allowing 2 inches leeway before the dome gets into the light path. On the assumption that your dome radius is around 40 inches, 2 inches corresponds to an angular distance of around 2.8 degrees, so you would need to set your allowable error to something less than that - I would suggest 1 degree for safety. That means that the dome will move more often but by smaller amounts to keep the scopes within the window of the slit. For a more complete answer, you would also need to take into account the field of view of your refractor, which could be quite wide for a 61 mm scope, and this will limit you further. For example, if your refractor has a 2 degree FOW, that’s a half angle of 1 degree, then your leeway drops down to only 1.8 degrees. Of course, if you are able to centre your scopes so that the outer edges are both about 8.7 inches on either side of the DEC axis, that would be the sweet spot if you can balance them.

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Hi There,
I have an SBS setup & by comparison my shutter width is fairly narrow. I can’t use more than one cammera at a time in SGP anyway so for now I just use different profiles with the appropriate slaving settings for each scope\camera setup.

@UrbanDome that makes sense and I was planning on just using one camera at a time anyway too. So having my slave settings setup for each scope since they are so unique would not be a big deal.

I have written an article that hopefully will be published this year in Astronomy Technology Today later this year and contains the maths required to calculate the dome azimuth for a SBS system with lateral offset. I can provide a copy of the article for implementation on request. A dedicated web-page is already on-line in which the algorithm can be tested.

NicolĂ s

That is some great work Nicolas. Guess it would be matter of the developers implementing the math/algorithm into SGP? Not sure where that would fall on the priority list or how complicated it would be.

It is not overly complicated and all very well explained in the article. For development I have made that web-page so that every step in the algorithm can be tested. Meanwhile a few programmers have tested my algorithm with success.

Personally I would certainly appreciate if the algorithm gets implemented into SGP, together with the option not to slave the dome during integrations (but in between them). My dome has a slit 80cm wide, but I have an Esprit 150ED and a Celestron C11 side by side with a third (60mm) scope in between. As a result the Esprit is regularly grazing the edge of the slit, while software like MountWizzard now allows me to go through the center, even with the C11.

NicolĂ s